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Greenstone belt
Greenstone belts are zones of variably to sequences with associated that occur within and s between and bodies. The name comes from the green hue imparted by the colour of the metamorphic s within the mafic rocks: the typical green minerals are , , and other green . A greenstone belt is typically several dozens to several thousand kilometres long and although composed of a great variety of individual rock units, is considered a ' grouping' in its own right, at least on al scales. Typically, a greenstone belt within the greater volume of otherwise homogeneous - within a contains a significantly larger degree of heterogeneity and complications and forms a tectonic marker far more distinct than the much more voluminous and homogeneous granites. Additionally, a greenstone belt contains far more information on and events, deformations and paleogeologic conditions than the granite and gneiss events, because the vast majority of greenstones are interpreted as altered s and other or s. As such, understanding the nature and origin of greenstone belts is the most fruitful way of studying geological history. Nature and formation Greenstone belts have been interpreted as having formed at ancient oceanic spreading centers and s. Greenstone belts are primarily formed of volcanic rocks, dominated by , with minor sedimentary rocks inter-leaving the volcanic formations. Through time, the degree of sediment contained within greenstone belts has risen, and the amount of ultramafic rock (either as s or as volcanic ) has decreased. There is also a change in the structure and relationship of greenstone belts to their basements between the Archaean where there is little clear relationship, if any, between basalt- sheets of a greenstone belt and the granites they abut, and the where greenstone belts sit upon granite-gneiss basements and/or other greenstone belts, and the where clear examples of volcanism, arc sedimentation and sequences become more dominant. This change in nature is interpreted as a response to the maturity of the processes throughout the Earth's geological history. Archaean plate tectonics did not take place on mature crust and as such the presence of thrust-in greenstone belts is expected. By the Proterozoic, was occurring around cratons and with established sedimentary sources, with little , allowing preservation of more sediments. By the Phanerozoic, extensive continental cover and lower heat flow from the mantle has seen greater preservation of sediments and greater influence of continental masses. Greenstones, aside from containing basalts, also give rise to several types of metamorphic rocks which are used synonymously with ' ' et cetera; , and are all terms spawned from the study of greenstone belts. The West African early greenstone belts are similar to the greenstone belts. These similarities include a decrease in the amount of and rocks as you move up the , in addition to an increase in s, and/or rocks. Also, the rock successions tend to have s in the upper portion and suites in the lower. s are common in these suites. Distribution in the of the }} Archaean greenstones are found in the , northern , and , , in , and in the in the US. Examples are found in South and Eastern Africa, namely the and also in the cratonic core of , as well as and , northern and the (see ). Proterozoic greenstones occur sandwiched between the Pilbara and Yilgarn cratons in Australia, and adjoining the Gawler Craton and within the extensive Proterozoic mobile belts of Australia, within West Africa, throughout the metamorphic complexes surrounding the Archaean core of Madagascar; the , northern Canada and northern Scandinavia. The in and is one of the largest Archean greenstone belts in the world. In Antarctica, the , of Proterozoic age, closely resembles the composition and structure of a greenstone belt. Greenstone belts often contain deposits of , , , and . One of the best known greenstone belts in the world is the South African , where gold was first discovered in . The Barberton Greenstone belt was first uniquely identified by Prof Annhauser at the , . His work in mapping and detailing the characteristics of the Barberton Greenstone belt has been used as a primer for other greenstone belts around the world. He noted the existence of s, indicating a lava being rapidly cooled in , as well as the textures created by s formed under rapidly cooling environments, namely water. List of greenstone belts Africa * (South Africa) * (South Africa) * (South Africa) * (Zimbabwe) * (East Africa) * (East Africa) * (West Africa) * (Burkina Faso) * (Burkina Faso) Asia * (Southeast Asia) * ( ), India * ( ), India * (Bundelkhand craton), India * , India Europe * (Russia) * (Lapland, Finland) * , Finland * (Norway) North America * (Quebec/Ontario, Canada) * (Manitoba, Canada) * (Wyoming, USA) * (Manitoba/Saskatchewan, Canada) * (Newfoundland and Labrador, Canada) * (Southwestern Greenland) * (Quebec, Canada) * (New Mexico, USA) * (Wyoming, USA) * (Wyoming, USA) * (Wyoming, USA) * (Ontario, Canada) * (Northwest Territories, Canada) South America * (Minas Gerais, Brazil) * (Minas Gerais, Brazil) * (Bahia, Brazil) * (Bahia, Brazil) * (Bahia, Brazil) * (Goiás, Brazil) * (Goiás, Brazil) * (Goiás, Brazil) * (Goiás, Brazil) * (Venezuela, Guyana, Suriname and French Guiana) Oceania * (Australia) * (Australia) * (Australia) * (Australia) * (Australia) * (Australia) References Category:Earth